1. Field of the Invention
The present invention relates to propellant measurement within propulsion systems. More specifically, the present invention relates to propulsion system propellant measurement in low gravity environments.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In spin-stabilized spacecraft the artificial gravitational field generated therein enables the employment of conventional fuel measurement schemes. For example, for liquid propellants the height of the liquid within the spacecraft propellant tank may be measured or the liquid pressure at the bottom of the tank may be gauged. However, conventional gravity based fuel measurement methods are inappropriate for thruster-stabilized spacecraft due to the absence of a gravitational field of sufficient magnitude.
Accordingly, alternative methods have been developed to estimate the amount of propellant remaining within spacecraft propulsion systems operative in low gravity environments. One such method includes monitoring changes in the absolute pressure within a propellant tank to thermodynamically deduce the volume of propellant remaining in the tank. Unfortunately, the narrow range of absolute pressures within the propellant tanks from the beginning to the end of a mission often results in unacceptable measurement error. For example, in the case of geosynchronous satellites up to eighty percent of the propellant carried thereby may be expelled simply to attain orbit. Thus, as little as twenty percent of the initial propellant load may remain at the onset of the useful life of the satellite. As the absolute pressure changes of the remaining propellant load are relatively small between the beginning and end of the useful life of the satellite, predictions as to the time of expiration of satellite propellant are prone to significant error.
A second approach to low gravity propellant measurement has been commonly termed the "bookkeeping" approach. Specifically, in the bookkeeping method the mass of propellant initially loaded into the spacecraft is recorded. As spacecraft thrusters are fired for attaining orbit and for station keeping the amount of propellant expelled is estimated. The amount of propellant remaining is simply the difference between that initially loaded into the spacecraft and that estimated to have been expelled. However, uncertainty with respect to temperature and pressure thrust parameters lead to errors in determination of the actual quantity of propellant consumed. Not surprisingly such errors tend to accrue over the useful life of the spacecraft--making predictions as to the end of the spacecraft life increasingly uncertain as time progresses. Further, the bookkeeping approach is incapable of accurately accounting for propellant leakage. In practice, the bookkeeping method of propellant measurement may yield end of life predictions in error by as much as one year on missions of approximately ten years. Hence, propellant measurement through both absolute pressure determination and bookkeeping schemes is subject to significant error.
The uncertainty in forecasting the probable time of spacecraft propellant expiration and termination of the useful life thereof tend to complicate mission planning. For example, replacement spacecraft typically need to be launched early enough so as to be operational and in orbit upon the termination of operation of the initial spacecraft. Hence if end of life predictions have an uncertainty of one year, replacement spacecraft may need to be launched a year in advance of the time launch would need to occur were the time of propellant expiration known with absolute certainty. As is well known, maintaining a replacement satellite in orbit during this uncertainty period is inefficient and tends to increase costs.
Hence, a need in the art exists for an accurate propellant measurement system capable of operation in a low gravity environment.